The present invention relates to a switched-mode power supply having an over power and over voltage protection circuit. In particular, the present invention relates to a switched-mode power supply comprising a primary circuit for receiving an input voltage, a secondary circuit for providing an output voltage, and a monitoring circuit for providing information relating to the input voltage in a first period of time and relating to the output voltage in a second period of time.
In general, electric equipment must be designed to operate under a broad variety of electrical environments. Power supplies for international applications are typically designed to accept line voltages/input voltages Vline in the range xcx9c85-276 Vac. This voltage span is required in order to cover most geographical areas including Europe and USA.
A widely used type of power supply is the switched-mode power supply shown in FIG. 1, where the input voltage/line voltage Vline is transformed to an output voltage, Vout,xe2x80x94the level of Vout typically but not necessarily being less than the level of Vline. The primary circuit 1 receives the line voltage Vline which, in a time phased way, is transformed to the secondary circuit 2 using the transformer 3. The transformer forms galvanic isolation between the primary and secondary circuits. In order to obtain a DC-like output voltage the secondary circuit typically includes means for rectifying the output signal. As shown in FIG. 1 the means for rectifying the output voltage may comprise a diode based rectifier and a capacitor.
The level of Vout is controlled by controlling the current in the primary circuit, Ip, using the controllable switch 4. Ip is determined by measuring a voltage drop across resistor 5. The measured voltage dropxe2x80x94which represents Ipxe2x80x94is provided as a control signal to a Pulse Width Modulator (PWM) circuit. The PWM-circuit adjusts the conduction time of the controllable switch 4 so as to obtain a predetermined current value. Typically, the controllable switch 4 is a transistor. The primary winding of the transformer has the inductance L. When the PWM-circuit switches the transistor on, Ip starts to build up in the primary circuit. The increase of Ip during ton (ton is the conduction time of the transistor) is illustrated in FIG. 2. FIG. 2 also illustrates the voltage across the controllable switch 4 and the current in the secondary circuit, Is. The dashed line shows Vline.
The controllable switch is switched off when Ip has reached a predetermined value. Thus, the conduction time is dependent on the predetermined level of the Ipxe2x80x94i.e. increasing the level of Ip increases the conduction time. For obvious reasons the conduction time is also dependent on the level of the input voltage and the inductance, L, of the primary winding of the transformer.
When the predetermined level of Ip has been reached the controllable switch 4 is turned off, and the magnetically stored energy in the transformer 3 is transformed to the secondary circuit 2. The transformation of energy to the secondary circuit induces a current, Is, in the secondary circuit. Is is rectified using e.g. the diode based rectifier and the capacitor 7 in combination.
The input power to the primary circuit is given by:
P=xc2xdLIp2f,xe2x80x83xe2x80x83(1) 
where L is the inductance of the primary winding, f is the PWM operation frequency (f=1/TPWM=1/(ton+toff+tvalley)) and Ip is the current in the primary circuit.
One problem that arises with prior art switch-mode power supplies is the fact that in order to over power protect a power supply at a fixed operation frequency, Ip must be limited.
Instead of operating the power supply with a fixed frequency, the prior art also discloses system operating with a variable operation frequencyxe2x80x94the so-called self-oscillating power supplies (SOPS). In SOPS""s the maximum power is not only dependent on Ip but also dependent on Vline because the operating frequency depends on Vline. This dependency is illustrated in FIGS. 3 and 4, where FIG. 3 illustrates the situation of a low Vline and FIG. 4 illustrates the situation of a high Vline. Comparing FIGS. 3 and 4 it is evident that the time required to obtain a predetermined value of Ip is shorter in the case of the high Vline. The shorter time required in FIG. 4 results in a higher operation frequency. In prior art SOPS""s a higher operation frequency means that more power is provided to the transformer. The increased amount of power provided to the transformer may reach damaging levels.
However, in the prior art, over power protection circuits for power supplies are known. Depending on the type of circuit being used, the prior art circuit shuts down the power supply, xe2x80x9ccrowbarsxe2x80x9d the faulty output, or switches the power supply to a different operating mode. A typical over power protection circuit according to the prior art is shown in FIG. 5.
It is a disadvantage of the prior art protection circuit shown in FIG. 5 that four additional external components (R1, R3, R4 and Z1) are required. Such components are difficult to integrate in an integrated circuit since they take up too much die area. In addition, such components increase the power consumption significantly and are susceptible for noise due to the high ohmic circuitry. Also, prior art arrangements as shown in FIG. 5 are less accurate.
It is a further disadvantage of the prior art power supplies shown in FIGS. 1 and 5 that in order to obtain information relating to the output voltage a separate feedback loop/control loop must be implemented. In case such a loop is broken no information relating to the output voltage is available, and thus, no protection against damages due to over voltage on the secondary circuit is available.
It is a still further disadvantage of the prior art protection circuits that at least one additional pin on the integrated circuit is required in order to get access to the external components.
It is an object of the present invention to provide an over voltage and an over power protection circuit for switched-mode power supplies. The invention is defined by the independent claims. The dependent claims define advantageous embodiments.
Advantageously, an integrated circuit is provided with an integrated over voltage and over power protection circuit without the use of additional die demanding external components. A consequence of this is the fact that no additional pins on the integrated circuit are required. Furthermore, external feedback loop/control loops can be avoided.
In principle, the energy storing device may be formed by a set of magnetically coupled coils. Preferably, the energy storing device may comprise a transformer. In that case the first coil may comprise the primary winding of that transformer whereas the second coil may comprise the secondary winding of that transformer.
The monitoring means may comprise a third winding of the transformer. This third winding may be galvanic separated from the first and second winding. The number of windings of the third winding may be considerable less than the number of windings of the first winding.
The switched-mode power supply according to the first aspect of the present invention may further comprise a control circuit for controlling the controllable current switching means. The control circuit may comprise a PWM-circuit operating the switch at a frequency between 25-250 kHz. The control circuit typically response to a control signal from the third winding. This control signal may comprise a information relating to the performance or status of the switched mode power supply. Preferably, the control signal relates to the input voltage to the power supply in the first period of time. In the second period of time the control signal relates to the output voltage of the power supply.
Since a plurality of information is provided via the same control signal the control circuit may receive that information via a single input pin.
The controllable current switching means forming part of the primary circuit must be able to handle the current flowing in the primary circuit. Suitable candidates may be selected from the group consisting of MOSFET transistors, Bipolar transistors, IGBTs or GTOs or any combination thereof. Preferably, the controllable current switching means is connected in series with the primary winding of the transformer. In order to determine the current flowing in this circuit a resistor is included within the circuit so that the current may be determined by measuring the voltage generated across this resistor.